formed in the testicles) and estrogens (β‐estradiol, formed in the ovaries) are important male and female sexual hormones. They bind intracellular receptors that, as transcription factors, control the expression of sex‐dependent genes (see Section 4.2).
Vitamin D. Vitamin D increases the calcium concentration in the blood and assists in the formation of bones and teeth. Vitamin D deficiency is known as rickets in children and osteomalacia in adults.
2.3 Structure and Function of Proteins
Proteins represent the most important tools of the cell (Table 2.2). They catalyze chemical reactions, transport metabolites through membranes, recognize other molecules, and can regulate gene activity. If we consider genes as the legislative branch, proteins then function as the executive branch (i.e. as the executing organs). Proteins are built according to the same principles in both prokaryotes and eukaryotes.
Twenty amino acids serve as building blocks for peptides and proteins, linked to one another by peptide bonds (Figure 2.6). Polypeptides, therefore, are polymers made from amino acids. Polypeptides are polar molecules, possessing a NH2 group (amino‐ or N‐terminal) on one end and a COOH group (carboxyl‐ or C‐terminal) on the other. The diverse tasks and functions of proteins result from different arrangements (sequences) of amino acids.
Figure 2.6 General structure of amino acids and peptides.
The 20 amino acids differ in their side chains (Figure 2.7). The functional groups of the side chains, which protrude from the α‐C atom, dictate the conformation and later functionality of the protein by molecular recognition or biocatalysis. Amino acids exist in two optical isomers: the D‐ and L‐forms. Polypeptides are composed exclusively of L‐amino acids. D‐Amino acids can be found in bacterial cell walls and in many antibiotics (gramicidin, valinomycin). Since proteases can only cleave peptides composed of L‐amino acids, the incorporation of D‐amino acids results in a certain protection from untimely degradation.
Figure 2.7 Structures of proteinogenic amino acids. (Cysteine muss zu den amino acids with apolar residues.)
The proteinogenic amino acids can be divided into different groups according to their functional groups and residues (Figure 2.7 and Table 2.4):
Amino acids with apolar, lipophilic residues.
Amino acids with polar but uncharged residues (i.e. with hydroxyl or amide groups).
Amino acids with acid groups that are negatively charged.
Amino acids with basic groups that are positively charged.
Table 2.4 Compilation and grouping of the proteinogenic amino acids: two types of abbreviations are recognized internationally, which either consist of one or three letters; the codons that represent the amino acids in the genetic code are also given.
| Classification | Symbols | Codons |
|---|---|---|
| Neutral and nonpolar amino acids | ||
| Glycine | Gly; G | GGA GGC GGG GGU |
| Alanine | Ala; A | GCA GCC GCG GCU |
| Valine | Val; V | GUA GUC GUG GUU |
| Leucine | Leu; L | UUA UUG CUA CUC CUG CUU |
| Isoleucine | Ile; I | AUA AUC AUU |
| Tryptophan | Trp; W | UGG |
| Phenylalanine | Phe; F | UUC UUU |
| Methionine | Met; M | AUG |
| Cysteine | Cys; C | UGC UGU |
| Proline | Pro; P | CCU CCC CCA CCG |
| Neutral and polar amino acids | ||
| Serine | Ser; S | AGC AGU UCA UCC UCG UCU |
| Threonine | Thr; T | ACA ACC ACG ACU |
| Tyrosine | Tyr; Y | UAC UAU |
| Asparagine | Asn; N | AAC AAU |
| Glutamine | Gln; Q | CAA CAG |
| Basic amino acids | ||
| Lysine | Lys; K | AAA AAG |
| Arginine | Arg; R | AGA AGG CGA CGC CGG CGU |
| Histidine | His; H | CAC CAU |
| Acidic amino acids | ||
| Aspartate | Asp; D | GAC GAU |
| Glutamate | Glu; E | GAA GAG |
The human body is capable of synthesizing some amino acids; others must be obtained
